CHICAGO – A major union's endorsement became a central focus of a debate between the Republican candidates for Illinois governor on Wednesday, prompting questions about the contenders' GOP credentials, accusations of "selling out" taxpayers and pointed exchanges over the role of organized labor in the state's politics.

State Sens. Kirk Dillard and Bill Brady, businessman Bruce Rauner and state Treasurer Dan Rutherford met in their second-to-last debate before the March 18 primary. The event came the same day that the American Federation of State, County and Municipal Employees Council 31, the state's largest public-employee union, endorsed Dillard, and as campaign finance filings showed labor sinking even more money into a multimillion-dollar anti-Rauner advertising campaign.

Rauner, a businessman from Winnetka who has repeatedly blamed "government union bosses" for Illinois' dismal financial situation, wasted little time in using the endorsement to question Dillard's allegiances. He noted AFSCME and two teachers unions that also are backing Dillard have been big supporters of Gov. Pat Quinn and other Democrats.

"Why are you running in the Republican primary?" Rauner asked Dillard. "I think you should be running in the Democratic primary."

Brady also got in on the attack, questioning Dillard's vote last year against a landmark pension overhaul that cut worker benefits and is opposed by the unions. The measure is estimated to save Illinois $145 billion over 30 years.

"Many people on campaign trail say you sold out the taxpayers to the unions," said Brady, of Bloomington.

As he has for weeks, Dillard defended his support, saying many union members are Republicans and that the GOP – which has struggled in Illinois – can't afford to "demonize" potential supporters. He insisted the unions knew he wouldn't be in lock step with them, but that they appreciated he was willing to sit down and hear their concerns.

"They're going to expect No. 1, respect," Dillard said. "My approach with unions will work. It's not a sledgehammer, 'My way or the highway.'"

The Hinsdale lawmaker said he voted no on the pension bill, after supporting similar measures, because it was rushed through the Legislature and it didn't set limits on how Quinn and the Democrat-controlled Legislature could use the annual savings.

Brady voted in favor of the pension bill. Rauner opposed it because he said it didn't save enough money, while Rutherford said he believed the courts will find it unconstitutional.

Brady also insisted that he was the only "reliable Republican" in the race, and criticized Rauner – who has spent millions on television ads in his first bid for public office – of trying to buy the race.

"The real question here is: Why should the voters of Illinois trust their vote with Bruce Rauner? Someone that they didn't even know four months ago," Brady said. "The three of us have a pretty open book. We've got a track record."

Rutherford, of Chenoa, largely stayed out of the evening's most tense exchanges. Asked at one point by a moderator if he wanted to jump in, he said no, adding "I'm having a great time." He also said he regretted that a federal lawsuit filed against him by a former employee who accused him of sexual harassment and political coercion – allegations Rutherford has denied – have distracted from more serious issues in the race and made his campaign more difficult.

AFSCME, which represents about 100,000 Illinois workers, noted Dillard's time in office and vote against pensions in its endorsement. The union also blasted Rauner, who leads in polls and fundraising.

"He [Dillard] rejects the demonization of public employees – the teachers, state police and correctional officers, caretakers for those with disabilities, and thousands of others who provide the vital public services that Illinois citizens demand," executive director Henry Bayer said in a statement.

"On the other hand ... Bruce Rauner is a billionaire CEO with no experience and little understanding of state government. He calls himself a 'political outsider' but has a long record of political cronyism and questionable business deals."

Rauner has dismissed those claims, saying he's a target because he is ahead.